1. Field of the Invention
The present invention relates to a fuel pump driving structure and an internal combustion engine and to an internal combustion engine equipped with the fuel pump driving structure.
2. Background Information
A conventional fuel pressuring apparatus for an internal combustion engine has been proposed which drives a high-pressure fuel pump with a pump cam provided on one end of a camshaft that extends in an axial direction (see Japanese Laid-Open Patent Publication No. 2003-184688). With this conventional apparatus, the pump cam can be supported in a cantilever fashion because the high-pressure fuel pump is arranged near an end wall of a cylinder head and, thus, the apparatus can be made more compact. However, in recent years, increasingly higher fuel pressures have been demanded of high-pressure fuel pumps in order to achieve improved fuel efficiency. Consequently, in order to improve the durability of the pump cam, there are a demand for the pump cam to be treated in a special quenching process and a demand for the pump cam to be made of a material having a high resistance to wear. Therefore, a structure in which the pump cam and the camshaft are fabricated as separate members and the pump cam is press fixed to the camshaft by press fitting has been proposed (see Japanese Laid-Open Patent Publication No. 2005-133618).
Since the pump cam and the camshaft are formed as separate entities, the pump cam can be treated with a special quenching process and the pump cam can be made of a material having a high resistance to wear so as to improve the durability of the pump cam. Additionally, the apparatus can be made more compact because the pump cam, the camshaft, and the cam journal can be arranged in close proximity to one another. However, since a diameter of the camshaft at a portion where the pump cam is press fitted onto the camshaft is limited by the size of the pump cam, it is necessary to design the diameter of the camshaft at the portion where the pump cam is press fitted onto the camshaft to accommodate the limitation. As a result, there are situations in which the strength of the camshaft is insufficient with respect to bending input from the pump cam.